The present invention relates to a process for the preparation of a highly anticorrosive surface-treated steel plate, especially a rust-proof steel plate suitable for a car body.
Recently, a demand for a steel plate excellent in the corrosion resistance as a steel plate for a car body has been increasing, and there is observed a strong tendency to use a highly anticorrosive surface-treated steel plate instead of a cold-rolled steel plate heretofore used.
As this surface-treated steel plate, there can be mentioned a zinc-deposited steel plate. However, in case of a steel plate of this type, in order to increase the corrosion resistance, it is necessary to increase the amount deposited of zinc, and this increase of the amount deposited of zinc results in degradation of the processability and weldability. In order to eliminate this defect, there have been proposed a steel plate deposied with an alloy of zinc with at least one element selected from Ni, Fe, Mn, Mo, Co, Al and Cr and a multi-layer deposited steel plate. These steel plates are advantageous over the zinc-deposited steel plate in that the corrosion resistance can be improved without degradation of the weldability and processability. However, when these steel plates are applied to a bag structure portion or bent portion (hemmed portion) of an inner plate of a car body, for which a high corrosion resistance is required, the corrosion resistance is still insufficient. As the steel plate having a highcorrosion resistance, there has been developed a rust-proof steel plate having a zinc-rich coating, as disclosed in Japanese Patent Publication No. 24230/70 or No. 6882/72, and as a typical instance of the rust-proof steel plate, there is known Zinchrometal. However, even in case of this rust-proof coated steel plate, peeling of the coating film is sometimes caused in the portion subjected to processing such as pressing, and this steel plate is still insufficient as a highly anticorrosive rust-proof coated steel plate satisfying requirements for a material of a car body.
As means for eliminating the foregoing defects, there have been proposed a composite coated steel plate comprising a steel plate deposited with a plating layer of the zinc type as a substrate and a double coating layer of a chromate film and an organic composite silicate film formed thereon and a process for the preparation of this composite coated steel plate in Japanese Patent Application Laid-Open Specifications No. 108292/82 corresponding to U.S. Pat. No. 4,411,964 and No. 224174/83. This composite coated steel plate is excellent over the conventional surface-treated steel plates in the corrosion resistance and processability. However, if this steel plate is used for a car body, the adhesion to a cationic electrodeposition paint is poor and the corrosion resistance of the coating is not satisfactory.
The inner surfaces of many parts of a car body such as a door and a fender are ordinarily coated with a cationic electrodeposition paint, and a topcoat is applied onto the cationic electrodeposition coating on the outer or inner surfaces of the car body, for example, on the inner surface of a hood or the like. The adhesion to such a multiple coating should satisfy conditions severer than those required for the adhesion to the single cationic electrodeposition coating, and these conditions cannot be satisfied by the conventional composite coated steel plate.
In connection with the outer surface of a car body on which a composite coating of cationic electrodeposition coating/middle coating/top coating, the coating finish (appearance) and the adhesion of the coating are considered more important than the corrosion resistance. For this reason, cold-rolled steel plates have ordinarily been used. Recently, however, it is required to improve the corrosion resistance of the coating also for the outer surface of a car body, and this tendency is conspicuous in Canada and the United States. A steel plate deposited with a zinc or zinc alloy plating is suitable for improvement of the corrosion resistance of the coating. However, the steel plate of this type is inferior to a cold-rolled steel plate in the coating finish (cratering resistance: formation of pinhole-like defects on the cationic electrodeposition coating is called "cratering phenomenon", the cause of which is not clarified, this cratering phenomenon is caused on a steel plate deposited with zinc or a zinc alloy and this cratering phenomenon is observed also after middle coating-top coating and has serious influences on the coating finish) and the adhesion to the coating [water-resistant adhesion: this property is evaluated at the test of the coating adhesion under wet conditions where a sample coated even with a topcoat paint is immersed in pure water (deionized water) for a predetermined time (about 5 to about 10 days), 100 cross cuts extending to the substrate (steel) and having sides of 1 to 2 mm are formed on the coating, an adhesive tape is applied to the cross cuts and peeled and the number of cross cuts peeled together with the adhesive tape is counted; the cold-rolled steel plate is excellent in this water-resistance adhesion and the steel plate deposited with zinc or a zinc alloy is inferior]. Accordingly, practical application of the steel plate deposited with zinc or a zinc alloy is difficult.
As the rust-proof steel plate for a car body, there have been used a steel plate having one surface deposited and a steel plate having a rust-proof coating formed on one surface (in each steel plate, the other surface is the surface of the cold-rolled steel plate). However, these steel plates fail to satisfy the above-mentioned requirement of the corrosion resistance for the inner and outer surfaces of a car body. Accordingly, development of a highly anticorrosive rust-proof steel plate having basic properties required for the inner and outer surfaces (corrosion resistance, processability and spot weldability for the inner surface and coating finish property, coating adhesion, corrosion resistance of the coating, processability and spot weldability for the outer surface) has been desired.
The present invention has been completed as the result of researches made under the above background, and it is a primary object of the present invention to provide a process for the preparation of a highly anticorrosive surface-treated steel plate which is excellent in the corrosion resistance and processability and which has a good adhesion to a coating, especially a cationic electrodeposition paint, and enjoys a high corrosion resistance of the coating.
Another object of the present invention is to provide a process for the preparation of a highly anticorrosive surface-treated steel plate which has a good adhesion to a multiple coating of at least two coating layers, to which a higher adhesion than to the cationic electrodeposition coating is required, while maintaining high corrosion resistance and processability.
Still another object of the present invention is to provide a process for the preparation of a highly anticorrosive surface-treated steel plate which suitably satisfies the requirements of basic properties for the inner and outer surfaces of a car body.